1. Field of the Invention
The present invention relates to a method of removing oxides, and more particularly, to a method of effectively removing oxides and with improved salicidation capability.
2. Description of the Prior Art
In the fabrication of semiconductor integrated circuits, a metal-oxide-semiconductor (MOS) transistor is fabricated by forming a gate structure on a semiconductor substrate first. Lightly doped drain (LDD) regions are then formed within the substrate on the two corresponding sides of the gate structure. Next, spacers are formed on the sidewalls of the gate structure and an ion implantation process is performed to form source/drain regions within the substrate by utilizing the gate structure and spacers as a mask. In order to incorporate the gate, source, and drain into the circuit, contact plugs are often utilized for interconnection purposes, in which the contact plugs are composed of conducting metals such as tungsten and copper. Nevertheless, the interconnection between the contact plugs and the silicon material of the gate structure and the source/drain region is usually poor; hence a silicide material is often formed over the surface of the gate structure and the source/drain region to decrease the contact resistance between the contact plugs and the gate structure and the source/drain region.
Today, the process known as self-aligned suicide (salicide) process has been widely utilized to fabricate silicide materials, in which a source/drain region is first formed, a metal layer comprised of cobalt, titanium, or nickel is disposed on the source/drain region and the gate structure, and a rapid thermal annealing (RTA) process is performed to react the metal layer with the silicon contained within the gate structure and the source/drain region to form a metal silicide for reducing the sheet resistance of the source/drain region. In addition, depending on different integrated circuit designs, an oxide layer, such as a resist protecting oxide (RTO), may be deposited on the silicon material before forming the metal layer, and an etching process is performed to remove a portion of the oxide layers, where the metal suicides will be formed, to form a patterned salicide block (SAB) to protect a portion of the gate structure and the source/drain region from forming the metal silicides. The patterned salicide block is finally removed.
In the conventional method of removing resist protecting oxide (RTO), a silicon substrate including resist protecting oxide (RPO) is loaded into dry or wet etching apparatus, which contains diluted hydrofluoric acid, for a predetermined time, and the silicon substrate is then moved out. Next, the silicon substrate is transferred into a metal sputtering apparatus for performing a self-aligned silicide (salicide) process. However, it takes longer process queue time for transferring the silicon substrate from the wet etching apparatus to the metal sputtering apparatus; therefore, the process throughput is greatly affected. Besides, the surface of the silicon substrate, where the resist protecting oxide has been removed, may be formed with a native oxide having a thickness of about 4 to 10 angstrom (Å) because of the silicon substrate contacts with oxygen and water in air. As a result, the adhesion capability between the metal layer, which will be formed in the following sputtering process, and the silicon substrate is affected, and then affect the formation of the metal silicide, i.e. the salicidation capability is poor.
Accordingly, there are plenty of patents that have disclosed how to remove native oxides using a dry etching process before the self-aligned silicide (salicide) process. Because the reaction chamber of the dry etching process can be combined with the reaction chamber of the metal sputtering process in one frame (or a vacuum cluster system), thus the contact chance of the silicon substrate, where the native oxides have been removed, with air can be avoided.
However, in the conventional methods of removing native oxides using the dry etching process, as the thickness of the native oxides increase, some residues, which are not etched completely, are found remained on the surface of silicon substrate after the self-aligned silicide (salicide) process. Furthermore, some incomplete metal silicides, where the metal silicide should be formed, are also found. Under the circumstances, the resistance of the integrated circuit is increased.
Accordingly, a method of effectively removing oxides with improved salicidation capability is provided to improve upon the deficiencies from the conventional methods.